chapotraphouse

To reduce spread of respiratory diseases, we need to understand the mechanisms of spread. There is strong and consistent evidence that respiratory pathogens including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), respiratory syncytial virus (RSV), influenza, tuberculosis, and other coronaviruses such as MERS and SARS-1, are transmitted predominantly via aerosols. Infected individuals, whether symptomatic or not, continuously shed particles containing pathogens, which remain viable for several hours and can travel long distances. SARS-CoV-2 is shed mainly from deep in the lungs, not the upper respiratory tract, and the viral load is higher in small aerosols (generated in the lower airways) than in larger droplets (generated in upper airways). Whereas large respiratory droplets emitted when people cough or sneeze fall quickly by force of gravity without much evaporation, those below 100 µm in diameter become (bio)aerosols. Even particles tens of microns in diameter at release will shrink almost immediately by evaporation to the point that under typical conditions they can remain airborne for many minutes. In contrast with droplet transmission, which is generally assumed to occur via a single ballistic hit, the risk of airborne transmission increases incrementally with the amount of time the lung lining is exposed to pathogen-laden air, in other words, with time spent indoors inhaling contaminated air.

Respiratory infections may theoretically also be transmitted by droplets, by direct contact, and possibly by fomites (objects that have been contaminated by droplets), but the dominant route is via respiratory aerosols. The multiple streams of evidence to support this claim for SARS-CoV-2 include the patterning of spread (mostly indoors and especially during mass indoor activities involving singing, shouting, or heavy breathing), direct isolation of viable virus from the air and in air ducts in ventilation systems, transmission between cages of animals connected by air ducts, the high rate of asymptomatic transmission (i.e., passing on the virus when not coughing or sneezing), and transmission in quarantine hotels when individuals in different rooms shared corridor air but did not meet or touch any common surface.

The certification of surgical masks for particle/bacterial filtering efficiency (P/BFE) does not reflect equivalence to respirators as the filtration is typically compromised by poor face seal. The ASTM F2100-21 P/BFE certification, for example, requires at least 95% filtration against 0.1-µm particles and at least 98% against aerosolized Staphylococcus aureus, but this is on a sample of the mask clamped in a fixture, not on a representative face. In terms of filtering aerosols, N95 respirators outperform surgical masks between 8- and 12-fold. The effectiveness of certified surgical mask material against transmission when used as a filter was demonstrated in a hamster SARS-CoV-2 model. Infected hamsters were separated from non-infected ones by a partition made of surgical mask material; when the partition was in place, transmission of SARS-CoV-2 was reduced by 75%.

In addition to protecting the wearer, respirators provide very effective source control by dramatically limiting the amount of respiratory aerosols emitted by infectious individuals. In one study, risk of infection was reduced approximately 74-fold when infected, and susceptible individuals both wore well-fitting FFP respirators compared to when both wore surgical masks. Figure 2 reproduces Bagheri et al.’s demonstration of the dramatic decrease in total inward leakage for different types of respirators and surgical masks.

Source: Masks and respirators for prevention of respiratory infections: a state of the science review

In case it's not obvious from this already, not all respirators have exhalation valves. Surely I don't need to provide a source for this statement, and if you don't believe me you can google this yourself.

Also, lmfao at you appealing to the CDC while contradicting their advice. According to the CDC themselves:

Respirators and masks may filter particles and block droplets when you exhale (breathe, talk, cough, etc.). This reduces the risk that a person with a respiratory infection will spread germs to others. If you have a respiratory infection, there’s a chance you could pass your infection to others. Wearing a respirator or mask reduces the number of germs that you exhale into the environment around you. This is one way to prevent the spread of respiratory germs from person to person, like wearing a mask to prevent the spread of the germ that causes COVID-19.

How Well It Protects You: NIOSH Approved elastomeric half-mask respirators (EHMRs) and elastomeric quarter-mask respirators (EQMRs) protect you against gases, vapors, and particles when equipped with the appropriate filter, cartridge, or canister.

How Well It Protects Others Around You: Some EHMRs and EQMRs, such as those without exhalation valves, filter the air you breathe out and you can use them to protect others around you. If the EHMR or EQMR do not filter the air you breathe out, you should not use them if your goal is to protect others around you.

How Well It Protects You: NIOSH Approved FFRs, such as N95 respirators, protect you against particles. They do not protect against gases or vapors.

How Well It Protects Others Around You: Some NIOSH Approved FFRs have exhalation valves that open to let air escape when you breathe out. This makes it easier to breathe and can make the respirator more comfortable to wear. An FFR with an exhalation valve may not protect others as well as one without a valve.

• Without exhalation valves: NIOSH Approved FFRs without exhalation valves filter the air you breathe out. You can use this type to protect others around you.
• With exhalation valves: If the NIOSH Approved FFR has an exhalation valve, some of the air will come out of the exhalation valve and reduce the level of protection to others. Wearing one of these will provide similar levels of protection to others as BFCs and some disposable face masks and cloth masks.

Source: https://www.cdc.gov/niosh/topics/publicppe/community-ppe.html